Field of the Invention
The present invention relates to a solar cell stack.
Description of the Background Art
In the epitaxy of III-V multi-junction solar cells, so-called metamorphic buffers are used in order to deposit on the metamorphic buffers high-quality semiconductor layers of materials with a different lattice constant than that of the substrate, or of layers located below the buffer. By means of the metamorphic buffer, a so-called “virtual substrate” is formed having a lattice constant different from that of the original substrate. As a result, it is possible to achieve greater latitude in the choice of materials, for example for the various elements of a multi-junction solar cell, and to improve the efficiency of the multi-junction solar cell.
It is desirable for the lattice constant of the metamorphic buffer to be increased in general during manufacture. By this means, most layers of the buffer are compressively strained, with the dislocations forming in a more homogeneous manner, and in particular fewer cracks arising, as compared to a tensilely strained buffer. Moreover, it is desirable for all layers of the metamorphic buffer to be transparent to light of certain wavelengths so that the light can be used in the other solar cells for photoelectric energy conversion.
Multiple solar cell stacks having a metamorphic buffer are known from “Comparison of arsenide and phosphide based graded buffer layers used in inverted metamorphic solar cells,” by A. Zakaria, Richard R. King, M. Jackson, and M. S. Goorsky in J. Appl. Phys. 112, 024907 (2012). In addition, solar cell stacks with metamorphic buffers as depicted in FIG. 4 of the present application are known from US 20130312818 A1. Metamorphic solar cell stacks are also disclosed in “Current-matched triple junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight,” by W. Guter, J. Schöne, S. P. Philipps, M. Steiner, G. Siefer, A. Wekkeli, E. Welser, E. Oliva, A. Bett, and F. Dimroth, in Applied Physics Letters 94, 223504 (2009).
Additional solar cell stacks with metamorphic buffers are disclosed in J. Schöne, dissertation entitled “Kontrolle von Spannungsrelaxation und Defektbildung in metamorphen III-V Halbleiterheterostrukturen für hocheffiziente Halbleiter-Solarzellen” [control of strain relaxation and defect formation in metamorphic III-V semiconductor heterostructures for high-efficiency semiconductor solar cells], 2009, Faculty of Engineering at Kiel University, Germany.
Furthermore, it is desirable in metamorphic buffers to relieve lattice strains through the formation of dislocations or other crystal defects in the buffers themselves, with the crystal defects remaining localized in the buffers as much as possible. In particular, threading dislocations should be prevented from propagating into other parts of the semiconductor layer stack. To this end, it is preferred to have the hardness of the buffer layers in the metamorphic buffers increase with the lattice constant, in order, in particular, to reduce the propagation of dislocations into layers located thereabove and/or to hinder the relaxation of layers located thereabove. In contrast thereto, it is disclosed by V. Klinger, T. Roesener, G. Lorenz, M. Petzold, and F. Dimroth, in “Elastische und plastische Eigenschaften von III-V Halbleitern für metamorphe Pufferstrukturen” [elastic and plastic properties of III-V semiconductors for metamorphic buffer structures], 27th DKGG Workshop, “Epitaxie von III/V-Halbleitern,” Erlangen, Germany, Dec. 6-7, 2012, that for a metamorphic buffer as shown in FIG. 5 of the present invention made of the ternary compound Al0.4InxGa0.6-x (0<x<0.6) in which the element gallium is successively replaced by indium, the lattice constant increases with the indium content, while the nanohardness decreases, shown as a solid line in the present FIG. 2.